Everyone with performance issues should first try lowering texture quality. I have a 1060 with 8GB of system ram and Insane ran horribly. They aren't lying about the ram usage requirements, for better or worse. I knocked it down one and it's basically fine. Effects can cause a brief dip, but that's it.
The symptom that I experienced was single player being mostly fine in the earlier levels, but with weird hitching. In multiplayer that weird hitching turned into HUGE frame drops and stalls. It was a texture setting issue. It just eats RAM and I think it's a Source problem.